#app .box {
  display: flex;
  width: 100px;
  height: 100vh;
}
#app .box .left {
  float: 1px;
  background: :red;
}
#app .box .mid {
  flex: 1;
  background-color: #ddd;
}
#app .box .right {
  flex: 1;
  background-color: aqua;
}
